The present invention relates to an apparatus for entrenching submerged elongate structures and, more particularly, to an apparatus for entrenching submerged pipelines.
In recent years, there have been many large oil and natural gas deposits discovered in off-shore locations. Subsequent drilling and production of these deposits and the need for an economical means for transporting the oil and gas obtained, from the production site to a collection terminal, has resulted in the need for long distance submerged pipelines.
Initially, pipelines were laid and left to rest upon the ocean floor. It soon became apparent, however, that serious damage was being inflicted upon the pipelines, such as for example, shifting of the pipelines due to currents, corrosion of exposed portions of the pipelines, and general structural damage caused by entanglement with marine equipment. In order to solve the problems encountered, it was found desirable to entrench the submerged pipeline beneath the surface of the ocean floor.
One device that has found some degree of success has been the submerged sled. The sled has a frame structure for straddling the pipeline and includes port and starboard pontoon runners. The system also includes port and starboard high pressure jetting nozzles for dislodging soil from the waterbed and forming a trench for the pipeline. The sled is pulled along the ocean floor by a surface vessel which also supplies the required high pressure fluid for the jetting nozzles.
While the general concept of a jet sled has shown promise, the specific burying sleds thus far developed have suffered from several basic problems.
One problem has been difficulty in forming a trench through regions of hard or tough clay formations within the ocean floor. Under such conditions it is desirable to remove the miniumum volume of material to form the trench of desired depth. Thus, a jet sled having vertically disposed jet legs designed to form a trench having a rectangular cross section is inefficient in such cohesive subsoils. One solution to the problem has been to form the jet legs having the jetting nozzles on a curved fashion to fit around and below the pipeline being laid. Details of this type of construction may be had by reference to Perot, U.S. Pat. No. 3,751,927.
While the above described arrangement decreases somewhat the volume of material to be removed to form a trench, a sled using such an arrangement cannot be readily removed or withdrawn from the pipeline under emergency conditions. Divers would be required to go down to the submerged location, partially disassemble the device and manually jet away the subsoil to permit the conduits to be freed from the pipeline. The use of divers is both time consuming, expensive and hazardous.
Further, prior burying sleds, though being adjustable, require extensive efforts to alter the depth to which a trench can be formed and the size of pipeline which can be accommodated. Adjustability of prior burying sleds has been accomplished by horizontal guide tracks which receive sliding mounts attached to the fluid conduits. When the conduits are in position around the pipeline, the structure is pinned by divers. An example of this approach to burying sled adjustability is that of the reference Perot, U.S. Pat. No. 3,751,927.
Prior entrenching devices have relied upon engagement with the pipeline to afford directional guidance. Such devices have used roller mechanisms which wrap around a major portion of the circumference of the pipeline and ride along the pipeline supported by the rollers. One such device is that described in U.S. Pat. No. 3,732,701, issued to Robert P. Lynch. Other devices, while not being supported by the rollers, do rely on pipeline engagement by rollers to guide the device. An entrenching device of this type is that shown in U.S. Pat. No. 3,583,170, issued to DeVries.
From the foregoing, it is apparent that it would be highly desirable to provide an apparatus for entrenching submerged elongate structures which would be capable of forming a trench particularly on cohesive clay-type subsoil with a minimum volume of material being removed and be operable to permit rapid removal off the elongate structure without inflicting damage thereto. Further, it would be desirable to provide an entrenching apparatus which does not ride upon the elongate structure to obtain directional guidance. Moreover, it is additionally desirable to provide a rugged burying sled capable of providing adjustment for various trench depths and various pipeline sizes.